Rev Bras Med Esporte vol.3 no.3 Niterói July/Sept. 1997

http://dx.doi.org/10.1590/S1517-86921997000300003

It is well known that strength training in younger
people and athletes results in significant physiological
adaptations and performance gains. For many years, however, this
type of exercise training was considered dangerous for older
men and women. Endurance training, on the other hand,
became an accepted form of physical activity in the elderly due to
its positive effects on cardiovascular function and health.
The myth that strength training is not useful and/or safe in
older adults has started to disappear. During the last ten
years, many scientific studies have shown that older adults can
be safely trained with strengthening exercises.
Furthermore, many investigations have demonstrated that strength
training results in significant physiological, functional, and
psychological benefits.

WHY RESEARCH IN AGING?

In the context of human evolution, aging appears to be
a recent phenomenon. During the 20th century a dramatic
increase in life expectancy and in the number and
percentage of people in the older age groups has been reported. In
1990, approximately 9% of the population of the world was 60
years or older and it has been estimated that by the year 2030
the proportion of people in this age group will increase to
approximately 16%. Despite the variability in
demographics among countries (in some countries approximately 20%
of the population already belongs to this age group), the
trend towards an older society seems to be universal. Thus, it
has been estimated that each month, the world sees a net gain
of 800,000 people over 65.

A FUNCTIONAL APPROACH

The problem, however, is not the statistical or
mathematical significance of the population changes. Our main
concern should be the functional implications of advanced
adult age. Aging is associated with loss of muscle strength,
muscle mass, motor units, aerobic capacity, hormonal reserve,
and many other physiological changes. Together, these losses
result in a reduction in maximum gait speed, ability to
perform activities of daily living, power to climb stairs or rise from
a chair, and other impairments and disabilities. As a result
of those physiological losses, an 80-year old person cannot
do those things that he/she considered easy at age 20. The
force required to complete certain tasks may represent, in the
elderly, a maximal or supramaximal effort when compared
to the same task performed by younger (and stronger)
persons. The end result of the reduction in physiological capacity
is deconditioning, loss of independence, and increased use
of medical services, all of which place a significant burden
on all individual governments and society in general.

AGING OR AGE-RELATED LOSSES?

Aging is difficult to define. Many statistical reports use
a specific age limit (usually 60 or 65 years) to classify the
population but it is clear that chronological age cannot be
considered a good index of physiological age. According to
the cell biologist Leonard Hayflick, age changes begin in
different parts of the body at different times and the rate of
annual change varies among various cells, tissues, and
organs, as well as from person to person. On the other hand,
the physiologist Paola Timiras defines aging as the sum of
all changes that occur with the passage of time. Thus,
aging does not seem to be neither a simple nor a uniform
process easy to define or study.

Of particular interest are observations that relate aging
to changes that usually characterize inactivity and/or
malnutrition. The sequence of events starting with physical
and biochemical changes in muscle and ending in functional
impairment and disability appear to be very similar in
these processes. Further, we know that elderly men and
women have a reduced level of habitual physical activity and
reduce their food intake. Perhaps, some of the changes usually
ascribed to aging are associated processes and not the end result of, or consequences of
aging.

STRENGTH TRAINING IN THE ELDERLY

If the assumption that some of the physiological losses
with age can be explained by associated processes is true, it
can be suggested that properly designed corrective
interventions, such as exercise training programs, may prevent some of
the losses and help with the recovery of functional capacity
in the elderly.

Interest in the strength of the aging skeletal muscle is
evident when we consider that the number of published
scientific studies on the topic has increased from 3 in the
1966-1974 period to 132 in the years 1994 to1997. Since 1988,
at least 25 studies have explored the physiological
adaptations and functional benefits of strength training of the muscles
of the lower limbs. An additional 6 studies have examined
the effects of strength training on muscles of the upper limbs.

In general these studies have included men and women
in the 60- to-90 yr-old age range and the training has
been based on the progressive resistance exercise model
which requires a progressive change (i.e., every week) in the
training load to adjust for the gains made during the
preceeding sessions. The purpose of this approach is to maintain
the training stimulus constant. An example of the exercise
prescription used in several studies is included in the
following table:

Programs similar to the one described above have
resulted in strength gains ranging from 15 to 175% of the
initial (pre-training) strength. A 10 to 15% increase in muscle
mass has been consistently reported based on measurements
using sophisticated imaging techniques (CT scan or MRI).
Individual muscle fibers (both type I and II) also show
significant hypertrophy (10 to 30%) as seen in cross-sections of
biopsy specimens stained with histochemical methods. It seems
that the process that leads to strength gains and hypertrophy
includes a dynamic turnover of muscle proteins and that
skeletal muscle in old age is capable of responding to the
stimulus provided by exercise with the synthesis of new
myofilaments. Finally, strength training has been shown to preserve
bone density while improving muscle mass, strength, and
balance in postmenopausal women. These observations have
particular clinical relevance given the high incidence of falls
in the elderly and the associated morbidity and mortality.

As reported in the younger population, the larger
relative increases in strength compared to the changes in muscle
size suggest a significant effect on the nervous components of
the neuromuscular system. The nature of the so-called
neural adaptations is not clear but adaptations in the nerve
conduction velocity, spinal reflexes, motor unit activation
and synchronization, and central cognitive-learning
processes may all play a role.

At least two studies have reported peripheral
adaptations in the oxygen transport chain resulting in small but
significant improvements in maximal aerobic power after
strength training. Both, capillary supply and the activity of
oxidative enzymes (citrate synthase) have been reported to
increase suggesting the adaptations in
VO2 max are peripheral in nature. Additional cardiovascular adaptations include an
attenuation of the blood pressure response to exercise when
subjects lift the same load after training. This adaptation
could reduce the stress on the cardiovascular system during
regular activities such as carrying objects at home or work.
Despite these positive observations, strength training should
not replace endurance training for developing aerobic power
and capacity.

Strength training seems to be a safe type of exercise
even for the frail elderly. The limits to strength training in
the older population are not well understood. Both, men
and women, respond to strength training and even
nonagenarians seem to retain the ability to adapt to this type of
exercise. Long-term studies (1-2 years) do not show a clear plateau
in strength gains. In other words, after many months of
training, volunteers continued to show improvements.
Finally, functional adaptations to strength training include
increased walking endurance, increased balance time, decreased
stair climbing time, and a reduction in the risk of falls. All
these have clear implications for achieving independence in life.